Reducing the atmospheric emission of controlled pollutants is a continuing societal concern. In the United States primary regulatory authority over industrial source air emissions resides with the U.S. Environmental Protection Agency (EPA). Over the years, the EPA has increased the stringency of air pollution control programs, both by decreasing the limits on acceptable emissions and by continually increasing the number and types of regulated pollutants.
As a source of controlled pollutant emissions, clinker kilns are subject to EPA emission regulations. Gaseous exhaust from a clinker kiln contains particulates and various gases. The particulates can be removed from the exhaust stream using a particle or dust separator such as an electrostatic precipitator (ESP) and/or a fabric filter baghouse (FFB) collector. The filtered exhaust gases can contain acidic controlled pollutants such as hydrogen chloride HCl and sulfur oxides SOx that must be reduced to meet regulatory requirements. In general, it is desirable that the concentration of hydrogen chloride HCl in clinker kiln emissions be less than 3 ppm (dry basis with 7% O2) and the permitted concentration of sulfur oxides SOx is set for each plant.
A wet chemical scrubber can be included after a dust filter; however, conventional methods to scrub acid forming gases such as hydrogen chloride HCl and sulfur oxides SOx can be expensive and inconsistent with the economic operation of an energy-efficient clinker kiln. For example, conventional wet scrubbers, which commonly use spray droplet sizes greater than 1000 microns, typically use 10 gal/min to 100 gal/min of liquid to scrub 1000 standard cubic feet per minute of effluent gases (1-10 kilograms of liquid per kilogram of gas). As a result, the consumption of water, scrubbing chemicals, and energy using conventional wet scrubbers is high.
Hot clinker kiln exhaust gases must be cooled to approximately 150° C. to have acceptable particulate emissions from an ESP or cooled to approximately 180° C. to protect a FFB from overheating. To cool and condition hot kiln exhaust gases, before entering an ESP or FFB a spray of water can be used to cool and condition the exhaust gases in a gas conditioning tower (GCT). Further cooling and conditioning may then be performed by passing the conditioned exhaust gases through cool wet limestone of a raw mill used to grind the raw materials into a kiln feed, which can then be transported to a pyroprocessing kiln. A GCT can also be used in parallel with the raw grinding transport if the moisture content of the raw material is high; however, a GCT is generally used in an upstream process when the moisture content of the raw material is low.
An approach to reduce hydrogen chloride HCl and sulfur oxide SOx emissions from a clinker kiln is to inject a high-pH lime slurry into the spray used in a gas conditioning tower (GCT). To prepare the lime slurry, lime can be reacted with water to form calcium hydrate Ca(OH)2 (hydrated lime). Hydrated lime Ca(OH)2 can be introduced into a GCT in dry form containing, for example, up to about 2 wt % water, or can be in the form of a wet slurry having a hydrated lime solids content, for example, up to about 50 wt % and introduced into the GCT as a spray of droplets. The lime CaO used to form the slurry is typically purchased from a supplier and transported to the clinker kiln, which increases the cost of reducing emission of controlled pollutants. The lime is typically produced to be highly reactive with acid gases, which can be determined by a number of factors such as the pore volume, the specific surface area, low water content, and low agglomeration. Alternatively, hydrated lime, which is stable and easier to handle, can be prepared remotely and transported to the clinker kiln. Ca(OH)2 in the lime slurry adsorbs and reacts with sulfur oxides SOx and other acid forming gases such as hydrogen chloride HCl to produce thermally stable salts, which can be filtered from the effluent gases and thereby reduce emissions of the acid forming species in the exhaust gases.
It is desirable to find alternative ways to reduce controlled acidic pollutants in clinker kiln emissions to meet the continuously evolving environmental standards.